Underbeings are humanoid individuals who have magical ancestors but have no magical powers (CG).
Underbeings with horns or strange-looking eyes worked as money-collectors for Circus Arcanus. Skender also referred to Maledictuses such as Nagini as Underbeings (CG).
The term indicates that such individuals are consider to inhabit a lowly place in an implied social hierarchy. It may be inspired by the Nazi term untermensch, which translates literally to "sub-man" and meant "inferior people." It was used in reference to all people who were not Aryan (source: Wikipedia).
It is unclear whether Squibs are considered Underbeings. When the term is introduced in CG, it is defined as "humanoids" who have "magical ancestry" but no magical powers. This seems like it could include Squibs, but the individuals deemed Underbeings in the stage directions have horns or otherwise do not quite resemble the typical human being. -BB
From the Web
Underbeing at Harry Potter Wiki